Method for eliminating metal halides that are present in a liquid or gaseous, organic or non-organic effluent

ABSTRACT

The invention relates to a method for eliminating metal halides which are present in a liquid or gaseous, organic or non-organic effluent. According to the invention, the elimination is carried out by absorption of said metal halides on alumina agglomerates. The inventive method is characterised in that: the specific surface area of said agglomerates is between 50 and 350 m 2 /g, preferably between 70 and 300 m 2 /g and, better still, between 80 and 250 m 2 /g; and the V 80Å  thereof is greater than or equal to 20 ml/100 g, preferably greater than or equal to 25 ml/100 g, better still greater than or equal to 30 ml/100 g and, optimally, greater than or equal to 35 ml/100 g.

The invention relates to the field of the elimination of impuritiescontained in organic or non-organic industrial effluents in the liquidor gaseous state. More precisely, it relates to the elimination ofimpurities consisting of metal halides contained in these effluents byabsorption on alumina agglomerates.

Many gaseous or liquid industrial effluents contain impurities that itis desirable to eliminate. These impurities may pose problems of varioustypes. Among these mention may be made of:

-   -   interference with processes in which the effluent participates,        for example formation of undesirable products or inhibition or        poisoning of a catalyst;    -   degradation of the quality of the final product resulting from a        lack of purity, from undesirable coloration, etc.; and    -   formation of industrial waste coming from treatment of the        effluent, said waste being difficult to reprocess and therefore        posing environmental problems.

It is known to improve the purity of certain liquid or gaseous, organicor non-organic industrial effluents by passing them over a mineralmaterial, such as alumina, that retains certain impurities by adsorptionon its surface. In particular, it is known to use alumina agglomeratesto purify industrial effluents containing metal halides in tracequantities that it is desired to eliminate. It is usually accepted thatthese alumina particles must have a high specific surface area andtherefore predominantly pores of very small size.

The object of the invention is to propose a method of purifyingindustrial effluents containing metal halides by selective adsorption onalumina, using forms of alumina that are relatively inexpensive tomanufacture but nevertheless are extremely effective for the applicationenvisioned, much more effective than the alumina particles for thispurpose in the prior art.

For this purpose, the subject of the invention is a method ofeliminating metal halides that are present in a liquid or gaseous,organic or non-organic effluent, in which this elimination is carriedout by adsorption of said metal halides on alumina agglomerates,characterized in that:

-   -   said agglomerates have a specific surface area of between 50 and        350 m²/g, preferably between 70 and 300 m²/g and even more        preferably between 80 and 250 m²/g; and in that    -   said agglomerates have a V_(80Å) of greater than or equal to 20        ml/100 g, preferably greater than or equal to 25 ml/100 g, even        more preferably greater than or equal to 30 ml/100 g and        optimally greater than or equal to 35 ml/100 g.

Preferably, said agglomerates have a V_(400Å) of greater than or equalto 10 ml/100 g, preferably greater than or equal to 15 ml/100 g and evenmore preferably greater than or equal to 20 ml/100 g.

Preferably, said agglomerates have a V_(37Å) of greater than or equal to45 ml/100 g and preferably greater than or equal to 55 ml/100 g.

Said agglomerates may include one or more dopant compounds selected fromcompounds of alkali metals, alkaline-earth metals and rare earths,having a maximum content of 20%, preferably less than 10%.

Said agglomerates may be in the form of beads, preferably having adiameter of less than or equal to 8 mm and more preferably between 1 and5 mm.

Said agglomerates may be in the form of extruded materials, for examplewith a cylindrical or polylobate shape.

Said extruded materials preferably have an inscribed diameter of theircross section of less than or equal to 4 mm.

Said alumina agglomerates may be in the form of powder.

In one particular application of the invention, said effluent is founddownstream of a polyvinyl chloride production unit. The medium may thenbe based on dichloroethylene and said metal halide is ferric chloride.

As will have been understood, the invention consists in using, asselective adsorbent materials, a class of alumina agglomeratesexhibiting particular characteristics in terms of both specific surfacearea and of porous structure. Surprisingly, in considering what hadpreviously been determined experimentally, these agglomerates must havea relatively low specific surface area and a porosity profile in whichpores of very small diameter do not necessarily represent a very largevolume. However, these agglomerates possess remarkably high adsorbentproperties with respect to metal halides contained in liquid or gaseous,organic or non-organic industrial effluents (it being possible, forexample, for the latter to be aqueous solutions).

The invention will be more clearly understood from the description thatfollows, given with reference to the single appended figure. This shows,for various alumina agglomerates according to the invention and variouscontrol alumina agglomerates, the degree of elimination (in percent) ofthe ferric chloride contained in a ferric chloride solution inacetophenone brought into contact with an alumina agglomerate after 37hours of reaction.

A preferential, but in no way limiting, application of the invention isdownstream of a polyvinyl chloride (PVC) production line. Afterpolymerization of the PVC, a residual dichloroethylene-based medium mayremain which contains traces of ferric chloride. This ferric chloridemust be eliminated before such a residual medium is recycled. It is alsoknown to carry out this elimination by adsorption on aluminaagglomerates such as those mentioned later with regard to the referencematerials. It will be seen that the use, for this purpose, of oneparticular class of alumina agglomerates substantially improves theefficiency of a ferric chloride elimination operation.

The alumina agglomerates used within the context of the method ofeliminating metal halides according to the invention must necessarilyhave a specific surface area of between 50 and 350 m²/g, preferablybetween 70 and 300 m²/g and advantageously between 80 and 250 m²/g. Alsonecessarily, they have a volume occupied by pores with a diameter ofgreater than or equal to 80 Å (denoted in short by the notation V_(80Å))of greater than or equal to 20 ml/100 g, preferably greater than orequal to 25 ml/100 g, advantageously greater than or equal to 30 ml/100g or even than greater than or equal to 35 ml/100 g.

It will be noted that the presence of pores having a diameter of lessthan 80 Å is of only little importance within the context of theinvention. The little importance ascribed to such microporosity goes, asmentioned, counter to what has been commonly accepted hitherto.

According to a preferred variant of the invention, the volume occupiedby the pores having a diameter of greater than or equal to 400 Å(V_(400Å)) is greater than or equal to 10 ml/100 g, advantageouslygreater than or equal to 20 ml/100 g.

The V_(80Å) and V_(400Å) may be determined by a conventional mercuryporosimetry method.

For this purpose, the alumina specimen is first placed in a column intowhich mercury under a pressure P is introduced. Since mercury does notwet alumina, its penetration or non-penetration into the pores of thespecimen having a given diameter depends on the value of P. In order tobe filled, the finest pores require a higher pressure P to beestablished than for filling the coarser pores. By measuring the amountof mercury penetrating the specimen for various values of P, it ispossible to determine the volume occupied by the pores of diametergreater than given values of this diameter.

According to one particular form of the invention, the aluminaagglomerates may be chemically modified by the addition of alkali metalor alkaline-earth metal compounds, or rare-earth compounds, or a mixtureof such compounds.

Preferably, compounds based on sodium, potassium, calcium, magnesium orlanthanum are chosen. Sodium is a preferred example, and may beintroduced in the form of one or more precursors of its oxide Na₂O.

The addition of one or more dopant compounds may be carried out beforeor after the forming operation or during it.

The dopant compounds are present in the alumina agglomerate with a totalmass content of less than 20%, preferably less than 10%.

These dopant compounds enhance the adsorbent properties of the surfaceof the alumina agglomerates with respect to the metal halide moleculesthat it is desired to eliminate.

The alumina may be used in powder form, but preferably it is used aftera forming step. Beads, advantageously with a diameter of less than 8 mm,preferably mostly between 1 and 5 mm, constitute a preferred form of thealumina agglomerates according to the invention. Another preferred formis that of cylindrical or polylobate extrudates, the inscribed diameterof their cross section preferably being less than 4 mm.

The beads may be obtained by means of a rotational technique, byagglomeration of an alumina powder in a pelletizer or drum. This type ofprocess makes it possible to obtain, in a known manner, beads withcontrolled pore distributions and diameters, these distributions anddimensions being, in general, created during the agglomeration step. Theporosity may be created by various means, such as the choice of theparticle size of the alumina powder or the agglomeration of severalalumina powders of different particle sizes. Another method consists inmixing with the alumina powder, before or during the agglomeration step,a compound, called a pore former, that disappears when it is heated andthus creates porosity in the beads. As pore-former compounds used,mention may be made, by way of example, of wood flour, charcoal, sulfur,tars, plastics or emulsions of plastics, such as polyvinyl chloride andpolyvinyl alcohols, naphthalene or the like. The amount of pore-formercompounds added is determined by the desired volume. One or more heattreatments then complete the operation of forming the beads.

The extrudates may be obtained by mixing and then extruding an aluminagel or an alumina powder or a mixture of various raw materials.

The initial alumina powder may be obtained conventionally by rapiddehydration of an aluminium hydroxide (for example, hydrargilite).

The addition of one or more dopant compounds may be carried out beforeor after the forming operation or during the latter.

As examples, the ferric chloride (FeCl₃) adsorption results for variouscontrol alumina agglomerates and for alumina agglomerates having thecharacteristics required by the method according to the invention willbe compared.

Eight alumina agglomerates were considered, the use of aluminas A, B andC forming a part of the prior art and the use of aluminas D, E, F, G andH corresponding to the invention.

100 ppm of FeCl₃.6H₂O were placed in a beaker containing 250 ml ofacetophenone. Next, 1 g of alumina pretreated at 300° C., in the form ofbeads or extruded materials, was then added. The beaker was thenisolated from the ambient air, shielded from light in order to avoid anydegradation of the solvent (which was photosensitive) and subjected tomagnetic stirring, the beads or extrudates being isolated from the barmagnet so as to avoid any undesirable attrition during the experiment.

The description of the alumina agglomerates used is given in table I, inwhich the diameters are in mm, the specific surface areas are in m²/gand the porosities are in ml/100 g. Table I mentions, apart from theabovementioned parameters, the V_(37Å) of each agglomerate, that is tosay the volume occupied by the pores having a diameter greater than orequal to 37 Å. The difference between V_(37Å) and V_(80Å) isrepresentative of the amount of pores with very small diameters of theagglomerate tested. Finally, the Na₂O content of the agglomerates,expressed in ppm, is mentioned. TABLE I Characteristics of the aluminaagglomerates tested Controls Invention Alumina A B C D E F G H FormBeads Beads Beads Extrudates Beads Beads Extrudates Beads Diameter1.4-2.8 1.4-2.8 2-4 1.2 2.0-2.8 1.4-2.8 1.2 2.0-2.8 Specific 337 257 6266 196 150 251 172 surface area V_(37Å) 35.6 37.2 53.5 68.4 68.0 102.359.8 58.4 V_(80Å) 14.1 9.4 53.4 48.9 57.5 98.5 41.5 46.5 V_(400Å) 5.34.6 53.2 5.8 20.0 53.7 5.4 15.2 Na₂O 3500 20000 700 500 700 700 2000020000

The iron chloride concentration of the acetophenone solution wasmonitored by UV-visible analysis, in particular by monitoring the changein absorbance at the wavelength of 378 nm.

After 37 hours of reaction at room temperature, the degrees ofelimination of iron chloride from the organic solution were thusdetermined, and these are plotted in the diagram shown in FIG. 1.

The aluminas used in the prior art show an adsorption potential markedlyinferior to that of the aluminas used in the method according to theinvention.

The ferric chloride adsorption efficiency of control alumina C, havingonly large-diameter pores and a very low specific surface area, is verymediocre.

It should be noted that the aluminas of the method according to theinvention have a specific surface area that is not particularly high: itis of the same order of magnitude or substantially less than that ofcontrol alumina B and substantially less than that of control alumina A.

Compared with control aluminas A and B, that have a similar or higherspecific surface area, the aluminas according to the invention aredistinguished by their relatively high V_(80Å). However, it should alsobe noted that, in many cases, the aluminas according to the inventionexhibit a small difference between V_(80Å) and V_(37Å), this being anindicator of the fact that they have few pores of very small size. Inparticular, this is the case for alumina F that gives the best ferricchloride adsorption results. However, it will be preferable for V_(37Å)to be usefully at least 45 ml/100 g, preferably greater than 55 ml/100g. It should also be noted that this alumina F has a high V_(400Å), andtherefore large-diameter pores present at a relatively high quantity.This all results in quite a low specific surface area, whichnevertheless does not compromise the quality of the results obtained,quite to the contrary. These results therefore go counter to what wascommonly accepted as having to constitute preferred characteristics ofthe alumina agglomerates in their envisaged application of adsorbingmetal chlorides.

Finally, it should be noted that the aluminas of the examples accordingto the invention do not have very high Na₂O (dopant compound) contents;they are at most 2%. Despite this, they exhibit ferric chlorideadsorption properties substantially higher than the control aluminas ofcomparable Na₂O content. This clearly shows that, for this application,the role of the porosity of the agglomerates is predominant, the dopingof these agglomerates by alkali or alkaline-earth metal compounds beingmerely a variant of the invention.

The invention is not limited to the specific examples that have beenmentioned, namely the adsorption of ferric chloride from an effluentbased on dichloroethylene or acetophenone. The use of aluminaagglomerates as described for the adsorption of metal halides may beenvisioned for the treatment of any gaseous or liquid, organic ornon-organic effluents. It is particularly applicable to aqueoussolutions.

1. A method of eliminating metal halides that are present in a liquid orgaseous, organic or non-organic effluent, in which this elimination iscarried out by adsorption of said metal halides on alumina agglomerates,characterized in that wherein: said agglomerates have a specific surfacearea of between 50 and 350 m²/g, preferably between 70 and 300 m²/g andeven more preferably between 80 and 250 m²/g; and wherein saidagglomerates have a V_(80Å) of greater than or equal to 20 ml/100 g,preferably greater than or equal to 25 ml/100 g, even more preferablygreater than or equal to 30 ml/100 g and optimally greater than or equalto 35 ml/100 g.
 2. The method as claimed in claim 1, wherein saidagglomerates have a V_(400Å) of greater than or equal to 10 ml/100 g,preferably greater than or equal to 15 ml/100 g and even more preferablygreater than or equal to 20 ml/100 g.
 3. The method as claimed in claim1 wherein said agglomerates have a V_(37Å) of greater than or equal to45 ml/100 g and preferably greater than or equal to 55 ml/100 g.
 4. Themethod as claimed in claim 1 wherein said agglomerates comprise one ormore dopant compounds selected from compounds of alkali metals,alkaline-earth metals and rare earths, having a maximum content of 20%,preferably less than 10%.
 5. The method as claimed in claim 1 whereinsaid agglomerates are in the form of beads.
 6. The method as claimed inclaim 5, wherein said beads have a diameter of less than or equal to 8mm, preferably between 1 and 5 mm.
 7. The method as claimed in claim 1wherein said agglomerates are in the form of extruded materials.
 8. Themethod as claimed in claim 7, wherein said extruded materials are ofcylindrical shape.
 9. The method as claimed in claim 7, wherein saidextruded materials are of polylobate shape.
 10. The method as claimed inclaim 7 wherein said extruded materials have an inscribed diameter oftheir cross section of less than or equal to 4 mm.
 11. The method asclaimed in claim 1 wherein said alumina agglomerates are in the form ofpowder.
 12. The method as claimed in claim 1 wherein-said effluent is adichloroethylene-based medium and in that said metal halide is ferricchloride.